I know that I have to re-enter my Simbrief Id into GSXPro but I'm not sure how to this.
GSX Manual, Page 32, the paragraph named "Simbrief Username", which is part of the chapter that explains all GSX Settings.
SimBrief username
In this editing field, you can insert your SimBrief username, to enable SimBrief integration for various operations in GSX, like setting the Passenger Number automatically, controlling the Refueling quantity, and have extra indication about the flight on the advanced VGDS systems, which support SimBrief integration.
The username you must type here is what is referenced in the SimBrief Account as the ALIAS.
It cannot contain spaces. While if your Alias was created a while ago, so it might contain spaces, SimBrief doesn’t allow to create an Alias containing spaces anymore, so GSX conforms to this standard. If your existing SimBrief Alias contains spaces, you need to edit on SimBrief account page to replace spaces with another character, for example an Underscore.
So, it must be your Username/Alias, which is a name, not the Pilot ID, which is a number.